1. Field of the Invention
This invention is directed to transducer devices for focusing energy of sonic, audible and ultrasonic waves in either transmitting devices such as loudspeakers and ultrasonic transmitters and receiving devices such as microphones and ultrasonic receivers. More particularly, the present invention is directed to transducers designed to maximize a sound radiating or receiving surface for focusing energy either being radiated from, or being received relative to, the transducers.
2. Brief Description of the Related Art
A variety of sonic energy transmitting and receiving devices are known which utilize thin film radiating elements. Such devices use the thin film to convert electrical signals into vibrations within a transmitting media such as in air, for air-coupled devices, or in water or other liquids, for liquid-coupled devices. The films may include, but are not limited to, metalized plastic films such as used in capacitance or electrostatic devices, piezoelectric plastic films used in piezoelectric devices, conductive films used in electromagnetic devices and the like. The total of the sonic energy generated or received by such devices is generally proportional to the surface area of the radiating element while a frequency and bandwidth of such devices is determined by properties of the radiating film as well as the structure of the backing element associated with the film. One known method of optimizing resonance frequency of such devices is to use a perforated type film and or to micro machine or texturize the backing surface for the film.
Cylindrical ultrasonic transducers are currently known which are specifically designed to focus sonic energy along a line of focus. However, the ability of such known devices to focus sonic energy into small regions is limited because of the geometric configuration of such devices. A total area of a radiating element of a cylindrical transducer device is proportional to the radius of the cylindrical configuration of the transducer. However, the total path length of the sound waves and, therefore, the attenuation of the sound within a coupling media, is also proportional to the radius of the cylinder. Because of the total area of the radiated energy from cylindrical transducer devices, most applications of such devices cause sonic waves to be reflected from a target surface without significant useful effect. This is especially true in the field of ultrasonic testing of laminate surfaces, package seals and the like.
Another limiting factor of known cylindrical transducer devices is the conformity of the radiating film element to the structure of the cylindrical shape of the transducer. Any deviation of the film radiating from the cylindrical surface can cause significant loss of efficiency. If there is more that 1/16 of a wave length of deformation between the radiating film and the cylinder, significant loss of efficiency occurs. This makes it only practical for relative low frequency use of such devices. Current methods of providing conformity between the sound radiating films and the cylindrical support surfaces of supporting electrodes include the use of adhesives or the use of mechanical applications of force around the periphery of the films or by the use of vacuum to draw the films to the cylindrical electrode surface.
For many industrial applications, it would be preferred to be able to focus sonic energy into a smaller area or a concentrated point than is possible using conventional cylindrical transducer type devices. To accomplish such focusing using thin film radiating elements, the radiating film would have to conform to a three dimensional surface such as a sphere which would be very difficult if not impossible to practice. Therefore, in order to create focused energy, known spherical or point focused transducer devices utilize solid radiating elements such as piezoelectric crystals. This makes such devices relatively expensive and limits the size of such transducers.
An alternate method of focusing sonic energy into a point focus is to use a flat radiating element and parabolic off-axis reflector. However, the total area of the reflector, and thus the radiating element, is limited by useful insertion angles of the sonic beam. The total path length of the sound waves and, therefore, the attenuation of the sound in a coupling media, is proportional to the focal distance and diameter of the reflector. The length of the sound path from each point on the radiating element to a focus point tends to be relatively long and further, there are generally parallel paths which extend from the radiating element to the reflector. Therefore, there is no energy concentration but only loss of concentration due to the attenuation in the media in which the transducer is functioning.
Yet another alternative method of focusing sonic energy into a point is to use a flat radiating element and Fresnel lens. The Fresnel lens could by a very thin element and therefore could operate without significant loss of energy due to attenuation in a media. However, a Fresnel lens works only for specific frequencies and thus is only practical for applications where very narrow bandwidth transmissions are used.
In view of the foregoing, there is a need to configure transducers which may operate either as transmitters or receivers in such a manner that sonic energy may be concentrated or focused into a point or other narrow area regardless of the media in which the transducer is used.